...do great speakers increase in value over time in the vintage market?

Hello to all...

I have a pair of JBL L25 "PRIMA" speakers, made 1972, case in 7/10 shape (not repainted), surrounds on woofers reformed several years ago, grill cloth replaced several years ago, includes JBL badges on speakers. Original 1972 price $169/ea. 

Recent searches of this speaker in the vintage market show prices 50% OR MORE than the origional issue price: ???

Is this that good a speaker design? - I have read comments saying it his/was one of the best speakers ever made by JBL (at that time?) - and I do luv their sound (which I now wonder if I've ever really heard them?) especially with well-recorded jazz and have the ability with little power to ROCK OUT!

(Would greatly appreciate any comments by those who have/had these over the years...)

DO GREAT SPEAKERS INCREASE IN VALUE OVER TIME, or do/should all depreciate in value...
The best example I can tell you of are GOODMANS OF ENGLAND - 
"AXIOM 80" Loudspeaker driver. We are talking mid 50's to very early 60's. Every senior loudspeaker engineer and audio entrepreneur from the early days of high quality audio equipment design and manufacturing, that I have ever had the pleasure of making their acquaintance, agrees on one subject when it comes to loudspeaker design.  The AXIOM 80 10" full range speaker was the most unique design ever invented. I could go on for pages describing the incredible sound produced with this driver. Unfortunately in those days, there we only a handful of companies worldwide that designed and manufactured complete loudspeakers packaged in carefully designed cabinets. Goodmans never attempted to go that far in the US market, only selling the drivers themselves.
The first time I heard one was around 1955 at MUSICRAFT on the north side of Chicago. One of my CSO associates took me up for a listen.
What I heard, considering the associated source electronics available in those days, came as close as I have ever heard to reproducing a violinist as if he were standing in the corner playing  live.
In 1956 at the Chicago Hi-FI show, I heard an AMPEX demonstration of 3 track tape. The loudspeakers they were using were 3 custom made enclosures, each mounting 4 axiom 80's, for each of the 3 channels.
To this day, I have never heard anything at any price that came as close to live performance. And, I have heard and owned a lot of equipment over the years
The following week I went out to MUSICRAFT and bought (8) eight AXIOM 80 drivers with the intention of building 2 of the enclosures heard at the AMPEX demo. The cabinet plans I acquired from GOODMANS of England. Unfortunately as circumstances would have it, the project never happened. A few years later I sold the drivers for $200.00 ea. Oh, and by the way, I still have the original purchase invoice. I paid $65.00 ea.
Now, to answer your question, I have rarely seen an axiom 80 driver on the market in recent times, that wasn't used and abused beyond usefulness. I have seen, on two occasions, (2) and also (4) AXIOM 80'S guaranteed to be in pristine condition. The photo's qualified the claims.  In one case, the asking price guaranteed and tested, was $3500.00 each. In the case of the (4) drivers, claimed to be mint, stored and never mounted. (Beautiful high quality photos), the asking price was $7500.00 each. This is just one example. I hope it answers and exemplifies your question.
High-Quality items have a better chance of retaining value. Speakers although a bit hit and miss still sometimes retain great resale. Tannoy, especially the 15in.I have a shedful of bits and pieces that follow my experience with hi-fi. I rarely sell anything and love hauling something out to renew my love of them. It's also great finding new combinations. It's not worth many clams but really enjoyable to buy a new item and find the best combination of speakers or cables or amps. If you've got the space and budget keep them and see if something new added to the equation renews the original attraction to them or even sight of them in a new improved light.
Douglas nailed it. $340 a pair in 1970’s dollars would equate to about 2k in todays money. $650 for a great example is on the high end and far below what they retailed for when you factor inflation. 
Goodmans did sell speakers in finished cabinets in the UK in the 50s.  My father had a single one (mono) he must have bought then.  He moved to stereo soon after so when I built my first stereo system in 1962 I purloined it and 'matched' it with a Wharfedale he also had in store that was taller and thinner with a darker cabinet.  Both two-way, the Goodmans sounded a lot better.  You could almost see the sides of the Wharfedale cabinet flapping.
I just restored a pair of AR 10 PI speakers ,new foam surrounds ,all new caps and resistors, new grills ,refinished cabinets,got the speakers cheap $200.00 ,total cost was $500.00, These sound great!!! I have a pair of Focal 1028 Be ,I used a pair of Rel 3Ti subs with both pairs ,both sound great but ,different, The ARs weigh 56lbs each .built very well, In 1977 they cost $900.00 for the pair today cost would be $3973.00,Not bad for $500.00